This invention relates to an improved charging device for rechargeable batteries and cells.
More particularly, this invention relates to an improved battery charger capable of very rapid and "gentle" charging of batteries without causing overcharge and overcharge-related effects such as battery overheating or shortening of cycle life.
Specifically, this invention relates to an improved battery charger which is capable of automatically determining a reference voltage and controlling the charging cycle accordingly to avoid overcharge.
Control of the charging process is achieved by periodically interrupting the charging current, determining resistance free voltage V.sub.o of the battery in fixed intervals after interruptions of current, and comparing the resistance-free voltage with a reference voltage V.sub.REF characteristic of the onset of overcharge reactions. The charging current is reduced as necessary, so that the resistance-free voltage does not exceed the reference voltage.
The reference voltage characteristic of the onset of overcharge reactions is determined during a constant current period of charging by determining the resistance free voltage V.sub.o of certain characteristic data points. The characteristic data points may be located on a charging curve, which relates resistance-free voltage V.sub.o to time t. With reference to FIG. 2, the characteristic points are:
1) the first inflection point I(1) of the charging curve V.sub.o (t), where the first derivative dV.sub.o /dt has a minimum, PA0 2) the point K of maximum curvature of the charging curve, also identified as the inflection point on the first derivative curve (dV.sub.o /dt), where the second derivative d.sup.2 V.sub.o /dt.sup.2 curve has a maximum, PA0 3) the second inflection point I(2) on the charging curve, where the first derivative dV.sub.o /dt has a maximum.
A suitable V.sub.REF is chosen as a function of either one, or a weighted average of two or more of the above characteristic points. For example, V.sub.REF may be computed by increasing values for V.sub.I(1) by a certain percentage, or by decreasing values for V.sub.k or V.sub.I(2) by certain percentage.
The rechargeable batteries and cells relevant to the present invention are of the type using nickel-cadmium (NiCd), nickel-metal hydride (NiMeH), leadacid and other chemistries. These are used in various applications ranging from small appliances like shavers, cordless power-tools, portable telephones, computers, toys, etc; as well as traction batteries for forklift trucks, golf carts and electric vehicles.
With the expected increase in reliance on electric vehicles, rapid and "intelligent" charging of such vehicles is a particularly important application because of the occasional need to extend its limited range by quick recharging on the road at an electrical "gas pump". Even when recharging the electric vehicle more slowly at a home charging location, it will be very important for the charger to avoid overcharging, which shortens battery life and which is characteristic of most currently used charging methods. An electric vehicle battery will represent a considerable investment and extending its life by using an intelligent charger will be required for economic feasibility. The term "intelligent charger" as used herein refers to the capability of the charging device to automatically determine the capacity of any subject rechargeable battery, and to control the charging cycle so as to reach maximum saturation without significant overcharge.
The charging of batteries involves forcing electrical current through the battery, usually under some control of the current (e.g. constant current) and often with some voltage control as well (e.g. maximum voltage). While there is some need for controlling the rate of the charging process itself, the most important need for control results from the need to stop the charging process when the battery becomes fully charged. After this point, continued charging of the battery leads to undesirable and wasteful overcharge reactions. Overcharge reactions in vented cells result in electrolysis and loss of water that has to be replaced; while in sealed cells it creates pressure and heat, as the recombination reactions of gases produced by overcharge reaction is exothermic.
Ideally, the overcharge reactions may require higher voltage and can be prevented simply by limiting the charging voltage to a certain value. This simple approach is, unfortunately, only partially successful with certain cell types, e.g. lead acid cells, vented NiCd cells, and sealed Li-ion cells.
Sealed cells capable of recombination of the overcharge reaction products (eg. NiCd cells) will usually tolerate overcharge at low rates, where the pressures of by-product gases are low and the heat generation is slow enough for the heat to be easily dissipated and lost. The need to stop the charging process when the battery becomes full is not too critical if no electrolyte constituent is being lost. However, continuous overcharge even at a low rate often reduces the cycle life of the cells.
Rapid charging, i.e. charging in less than one hour, presents much more of a challenge with both vented and sealed cells. The first problem results from the limited rate of charge distribution or equilibration within the electrode plates, so that some pans of the active mass which are electrochemically more accessible become fully charged and driven into the overcharge reaction, while some other pans of the active mass are not yet fully charged. The generalized charge acceptance curve in FIG. 1 shows that this problem is aggravated by increasing currents. That is, at higher charge rates the overcharge reactions will begin to show at a lower fraction of full charge. In the overcharge region the current efficiency of the charge reactions is declining and most of the coulumbic energy is being wasted on the overcharge reactions. To complete the charging process under these conditions one has to tolerate the overcharge reactions at the given rate for a sufficiently long time. In practice, this is often the case. As a result, the fast charge NiCd cells are more strongly catalyzed to prevent excessive pressure build-up during the overcharge period. This approach using a limited overcharge period is popular as it permits using simple charging technology developed for medium charging rates (1-6 hrs). However, the rapid heating of a battery during high rate overcharge cannot be avoided. Other problems include the possibility of exceeding a safe pressure, and cell venting, especially at lower temperatures, when the recombination catalyst is not as effective.
The methods used to terminate the rapid charge/overcharge process at the desired point are best discussed with the help of charging voltage curves illustrated in FIG. 2. Reduced and oxidized forms of active mass, Ni(OH).sub.2 and NiO(OH), will be designated as M.sub.red and M.sub.ox, respectively.
The first section A of the voltage curve V.sub.o /t represents the initial rapid voltage increase of a deeply discharged cell due to the first production of M.sub.ox, followed by voltage profile flattening during the main charge period, where there are sufficient amounts of both M.sub.ox and M.sub.red forms of the active mass. The second part B of the curve starts at the first inflection point I(1) and reflects the voltage increase due to the onset and then gradual increase of the first overcharge reaction. Part C starts at the inflection point I(2), where the overcharge reaction begins to dominate. The curve is separated at point K, the point of maximum curvature, so that part C can reflect differences between vented cells C.sub.v and sealed cells C.sub.s. For vented cells, the voltage curve at C.sub.v finally flattens to a plateau corresponding to the first overcharge reaction. In sealed cells with recombination, the overcharge reaction results in a noticeable increase in temperature starting at the first inflection point and becoming very visible after the second inflection point I(2). If cell voltage has a negative temperature coefficient (e.g. NiCd batteries), a peak P instead of the plateau will result from the rapid temperature increase in this part of the charging curve at C.sub.s.
One popular charge control system for sealed NiCd batteries is based on stopping the charging current after detecting the voltage peak. Some overrun, e.g. 10 mV, is necessary to distinguish between the real peak and the noise of the voltage readings. This "negative delta V" method works well for charging rates of about one hour. Of course, significant overcharge cannot be avoided by this method, as it depends on the effect of overheating caused by overcharging.
More sophisticated methods use detection of the second inflection point I(2) to stop the charging process. This method permits reducing the charging time to 15 minutes. Some overrim of the inflection point is again necessary, and overcharge is not totally avoided.
Both methods depend on crossing the charge/overcharge line in FIG. 1 to finish the charge at high rate, and will cause some unnecessary and deleterious heating of the battery. Because most batteries heat during discharge due to the thermodynamic and irreversible heat effects, additional heating on charging will result in rapid overheating of the battery in heavy use.
A much more logical approach to rapid charging is reducing the charging rate at the point where ;overcharge reactions begin to appear, which requires essentially following the charge/overcharge boundary line in FIG. 1. In this way, overcharge reactions are avoided, pressure increase is very low and the cell does not heat up due to the recombination mechanism of the overcharge reaction. NiCd cells with low internal resistance can even cool due to the endothermicity of the charge reaction.
One practical way of determining the onset of overcharge reactions is based on measuring resistance-free (open circuit) voltage during short but frequent interruptions of the charging current and comparing it to an external preselected reference voltage typical of the onset of overcharge reaction. When the sensed resistance-free voltage reaches this preset value (compensated for temperature), the current is gradually reduced so that the reference voltage is never exceeded. This method is described in U.S. Pat. No. 5,179,335 issued Jan. 12, 1993.
This method of "tapering" the current based on reading the resistancefree voltage of the battery works very well with certain vented and sealed batteries. Low resistance sealed NiCd batteries can be charged in as little as 5 minutes with less than 10.degree. C. temperature rise, or in 15 minutes with a temperature decrease. Conventional lead-acid batteries can be charged in 20 minutes, and aircraft-starting vented NiCd batteries can be charged in 15 minutes.
This method, however, does have significant disadvantages compared to the voltage curve method described above. Namely, the reference voltage used to control the charging process along the charge/overcharge line in FIG. 1 depends on the number of cells in the battery, on temperature, and to some degree on cell construction. While it is not especially difficult to set the proper V.sub.REF for the number of cells and their temperature, incorporating the effects of individual cell construction to the value of V.sub.REF is more complicated.